I would think it might be wise, if nothing else, to go through the inbox on a daily basis and answer the emails from people who have already paid thousands of dollars to you and sent you their valuable parts for re-work?
Even a "sorry it's been really hectic but I haven't forgotten about you" goes a long way, instead of just no reply, at all.
EDIT - haha wouldn't you know, he got back to me this morning
It would seem John is alive and well.